A first type of plant is that described in Italian patent IT1317056. This plant has been designed in order to implement a relatively complex waste treatment method. It is therefore characterized by a succession of apparatuses, each of which is designed to perform a specific function within the framework of the overall method. In this plant the municipal solid waste (MSW) is converted into so-called Refuse-Derived Fuel or RDF. This known type of plant, although very appreciated owing to the quality of the finished product, is not without drawbacks.
A first series of drawbacks consists of those associated with the complexity and therefore with the delicate nature of the waste treatment method. In particular a weak point of the plant has been identified in the counter-rotating blade mill, operation of which is easily affected or prevented by material which is difficult to grind. During the treatment of municipal solid waste, despite recent legislation aimed at ensuring the recycling or alternative disposal of special waste, it is not possible to exclude the presence of bodies which have a very strong structure, typically mineral or metallic bodies which are non-magnetic (and therefore cannot be eliminated by the devices usually situated upstream of the grinding stage, such as the so-called metal separators). The presence of such bodies prevents correct operation of the counter-rotating blade mill and therefore of the entire plant described in IT1317056. Whenever such an event occurs it is therefore required to stop the whole plant and maintenance personnel must intervene in order to remove the bodies which cannot be ground.
A second series of drawbacks associated with this type of plant is that of the overall energy consumption which is required for the entire processing operation. This energy consumption may be quantified at a figure of more than 250 kW for each tonne of waste processed. This figure is relatively high, in particular in view of the fact that it is required to add the further energy needed to remove, before loading the machine, all those components which may create problems (typically metal and mineral masses of any size) and finally to reduce the particle size of the material. The RDF discharged from the plant is in fact composed of parts which have a particle size in the region of 25-30 mm. which is too large for direct fuelling of a burner if the RDF is not combined with a larger quantity of another fuel, typically a fossil fuel. As things stand at the moment, therefore, the RDF produced by the plants of the known type, in order to be able to ensure effective combustion must be used in quantities of between 25% and 35%. Alternatively, said RDF could be further reduced in size in order to achieve a particle size of about 5-10 mm, with a further increase in the energy consumption, thus further reducing the overall energy efficiency of the processing method.
In addition to the drawbacks mentioned above, a further drawback has been encountered: the presence in the MSW of bodies which cannot be ground results in the use of a large amount of mechanical energy which, when protracted over time up to the removal of such non-ground bodies, results in a local increase in temperature. Within the mass of the MSW being processed, which on the whole remains at a temperature close to room temperature, some points may therefore reach temperatures which are much higher, even of the order of hundreds of degrees Celsius. These temperatures may easily produce softening of the polymer fractions present in the MSW and, eventually, blockage of the output grilles for the ground waste.
A second type of known plant is that described in the patent document EP2062645A1. This plant has been specifically developed for the treatment of so-called Waste of Electric and Electronic Equipment (WEEE). It comprises a mill consisting of a grinding chamber inside which a rotor operates. The rotor comprises a hub to which some chains are connected. The rotation of the hub causes rotation of the chains which, subject to the centrifugal force, are arranged radially and sweep the grinding chamber. The WEEE, introduced from above, is struck by the chains and is subject to a series of impacts and rebounding movements which cause it to be gradually broken up.
The use of this type of mill has proved to be relatively efficient only in connection with the WEEE for which it has been designed. Generally such waste has a fairly rigid structure which therefore gives rise to elastic collisions and, following more violent impacts, to elastic-brittle fractures which absorb a low amount of deformation energy. Owing to these characteristics of WEEE, in a short amount of time a large number of knocks and impacts are produced, resulting in an efficient breaking down of the material to an acceptable particle size.
The use of this type of mill has, however, not proved to be suitable for other types of waste, typically MSW and similarly processable waste (referred to below overall as MSW in short). Said waste in fact has a structure which, although it cannot be easily defined, overall has a very different behaviour in relation to the impacts, compared to WEEE. The mass of MSW in fact has an elasto-plastic behaviour or even a visco-plastic behaviour when there is a significant wet fraction. Such a behaviour results in collisions which are mostly inelastic and which absorb a large quantity of deformation energy. In other words, the MSW, introduced from above into the mill, is struck by the chains and, without any rebounding action, adheres to them and simply starts to rotate. The overall primary effects of this behaviour of the MSW consist in long dwell times inside the grinding chamber and high energy consumption due to the fragmentation process which is achieved by means of successive tearing produced by friction. Alongside these drawbacks there is at least one other drawback resulting therefrom. The long dwell time of the MSW inside the grinding chamber and the large amount of mechanical energy absorbed by it result in a general increase in the temperature of the mass being processed. This increase in temperature may easily result in softening of the polymer fractions present in the MSW and, in this case also, in the blockage of the output grilles for the ground waste.